Traditional plasmonic materials mainly composed of noble metal nanostructures, which has been of great importance in creating high performance biosensors due to its sensitivity to changes in the surrounding medium and the generation of resonantly enhanced nanoscale light fields. However, the fixed and relatively large free charge concentrations in these noble metals result in great damping losses and prohibit its potential implementation in future on-chip plasmonic sensing systems. More importantly, their bio-sensitivities are only limited to the biochemical events which induce significant change of the ambient refractive-index. It is beyond its capability for many others which are mostly charge-transfer-mediated.

Degenerately doped semiconductor are an emerging class of plasmonic materials for which plasmonic features can be controlled by the concentration of dopants in the crystal structure. This unique feature can offer new dimensions for plasmonic biosensing by allowing charge based detection. However, their operating wavelengths are limited to the range between near-infrared (NIR) and far-infrared (FIR) that is not easily accessible.

Here, we introduce a new class of tunable plasmonic materials based on either sub-stoichiometric or heavily doped two-dimensional (2D) molybdenum oxides. Due to their capabilities for accommodating a large number of vacancies and ions, their plasmonic properties can be tuned across the whole visible spectrum and the near-red end of the NIR region. Furthermore, through the demonstration of a few representative biosensing models, the bio-sensitivities of these materials are impressive as the structural vacancies and inserted dopants are facilely exchanged from the 2D planar host during biochemical events, providing an unprecedented opportunity in developing high performance biosensors.